Processes for the preparation of polyesters by polycondensation of bivalent phenols with chlorides of aromatic dicarboxylic acids, like phthalac acid, isophthalic acid and terephthalic acid, have been known for a long time. Various bivalent phenols, such as hydroquinone, resorcinol, dioxydiphenyl-methane, 2,2-bis-(4-hydroxyphenyl)-propane, phenolphthalein, and others can be used. All these polyesters have a rather low temperature stability, so that they can not be used at temperatures of for example, 300.degree. C. and above.
A similar process for the preparation of polyesters which can withstand temperatures of 300.degree. C. is also known. According to this process, 9,9-bis-(4-hydroxyphenyl)-fluorene is used as a bivalent phenol, either alone or in admixture with other bivalent phenols. These polyesters can be prepared both by two-phase-boundary surface-condensation and by polycondensation in high boiling solvents, as well as by low temperature polycondensation in chlorinated hydrocarbons by using a suitable HCl-acceptor. If 9,9-bis-(4-hydroxyphenyl)-fluorene is used primarily in these syntheses, polyesters are obtained whose softening temperature is, any case, above 300.degree. C. and with regard to which a true softening temperature below the decomposition temperature no longer takes place. This is particularly the case when 9,9-bis-4-(hydroxyphenyl)-fluorene is used exclusively as a phenol component.
By partial substitution of 9,9-bis-(4-hydroxyphenyl)-fluorene by other bivalent phenols, such as 2,2-bis-(4-hydroxyphenyl)-propane, the processing possibilities can generally be extended. For example, with a 50% use of 2,2-bis-(hydroxyphenyl)-propane in the phenol component, polyesters are obtained from which homogeneous molded articles can already be obtained at substantially lower temperatures than is the case with polyesters produced by the sole use of 9,9-bis-(4-hydroxyphenyl)-fluorene.
In judging the degree of flammability of plastics, it is customary to measure the so-called "limiting oxygen index" (LOI) according to U.S. specification ASTM D 2863, where the minimum concentration of oxygen in a streaming oxygen-nitrogen mixture is determined at which a combustion with the formation of flames can just be maintained in a standardized measuring arrangement. It has been found that the highest LOI value is obtained in the above-mentioned polyesters produced with 9,9-bis-(4-hydroxyphenyl)-fluorene, when the polyesters are produced almost exclusively with the use of this phenol. Depending on the chemical composition, type and shape of the test piece, LOI values of from 35 to 38% can be obtained. A small portion of 10 mol% 2,2-bis-(4-hydroxyphenyl)-propane in the phenol component results in a reduction of the LOI values to from 31 to 34% in the polyesters produced.
But even LOI values of 35-38% are at the lower limit of the admissible values for particularly critical application where a particularly low flammability is required, such as in airplanes. Other high-temperature resistant plastics, such as bis-maleimides or polyimides frequently have substantially higher LOI values, namely up to about 48%. However, during thermal decomposition of these plastics, toxic nitrogen compounds are formed, which sets certain limits to their use in the interior decoration of airplanes and other vehicles used for human transportation.